At present, processing organic waste with moisture content in a range of 75% to 85% is a serious problem. Although there are various heat treatment methods, these methods require preliminary drying of the waste which consumes quite large amount of energy. Energy saving and waste reduction can be achieved for satisfying environmental friendly conditions by processing the material mentioned above using heat radiation energy from the microwave dryer.
Said microwave is electromagnetic wave in frequency of 300 MHz to 300 GHz and having wavelength of 1 mm to 1 m. The principle of drying is that the microwave generator radiates microwave to a material to be dried, water molecules in the material would rotate and vibrate synchronously in accordance with frequency of the microwave when the microwave is injected to internal of the material, as the water molecules in the medium material being heated are polar molecules. Under rapid change of electromagnetic field operating in high frequency, orientation of the polar molecules would change according to the change of the external electromagnetic field, resulting in movement of the polar molecules and friction between the polar molecules. In this moment, microwave field energy can be converted into heat energy inside the polar molecules, such that temperature of the internal and surface of the material can be risen at the same time, with a result of a series of physiochemical processes (for example heating and expansion effects etc.), so that large amount of water molecules would be evaporated out from the material to achieve purpose of microwave heat drying.
Microwave heating facilitates an object being heated to be a heating element itself, which is called an overall heating. Therefore, uniform heating can be achieved within a short period of time, while heat conduction process is not required. As such, this characteristics can facilitate the material of poor heat conductivity being heated and dried within a short time, thus energy utilization rate is improved and the size of furnace can be smaller than a conventional furnace. At the same time, the overall temperature of the material is risen when the material is under the microwave electromagnetic field. In this moment, the temperature of the surface of the material would decrease as water on the surface is vaporized, which would result in a temperature gradient of higher temperature in internal of the material and lower temperature in external of the material. The direction of this gradient is the same as the water vaporization. Therefore, the efficiency of microwave heating is extremely high.
A Chinese patent application with application number 200610048560.X (Publication Number is CN101122440A) discloses self-flowing microwave heating dryer. The dryer does not have any leakage of microwave. A body of the dryer is a self-flowing pipe having an inlet at its top and an outlet at its bottom, a feeding funnel device is configured at the inlet of the pipe, a discharging device is configured at the outlet of the pipe, the discharging device is in a form of rotational impeller and prevents leakage of microwave, at least two sets of microwave drying devices for microwave radiation drying of the material passing through the pipe are further equipped in series on the pipe. It can be seen that the dryer of this invention can be used for drying (heating) material in particle or powder form. A vertical (tilt) configuration of the material pipe can arrive at automatic self-falling down of the material inside the device. Such configuration is characterized in simple structure, ease of control of flow rate of the material, higher uniformity of drying and better effect of energy saving.
A Chinese patent application with application number 02100566.4 (Publication Number CN1436996) discloses an air blowing microwave drying device comprising a microwave drying furnace, a layer of polytetrafluoroethylene (PTFE) is coated on four walls of cavity of the furnace, a bottle of drying container is placed on a rotational disc at the bottom wall of the microwave drying furnace, the top part of neck of the container is extended out of the microwave drying furnace through an aperture on the top wall of the microwave drying furnace. A microwave suppressor is arranged respectively on the contact point between the neck of the container and the top wall of the microwave drying furnace and a port at the upper end of the neck of the container. Micropores are arranged on the surface of the end of the microwave suppressor. A gas distribution panel is installed at the bottom of the bottle of the drying container, the panel is configured horizontally and has micropores. An inlet pipe passes through in an order of the microwave suppressor, the gas distribution panel and the bottom of the bottle of the drying container. The power of the microwave drying furnace is 200 to 700 watts (W). This invention can timely get rid of water vapor and heat energy generated during drying process to avoid overheating of the internal of the material due to accumulation of water vapor and heat, such that quality of the dried material can be guaranteed.
A Chinese patent application with application number 200610160006.0 (publication number CN101210771) discloses a microwave drying apparatus, wherein an annular box is configured on a rack, there is a fixed panel extending more half of an annular chamber in cross-section inside the box, at least 9 sets of microwave waveguide devices and microwave generators are configured on the fixed panel, the microwave waveguide devices are uniformly distributed with staggered arrangement on the fixed panel, under the fixed panel is a microwave drying chamber, a heating rotational device is configured beneath the microwave drying chamber, the heating rotational device consists of a variable motor and a heating rotational disc, a moisture discharge mechanism is arranged at the middle part of the annular box, and left and right suppressors and the material loading area are arranged along the less than half of the annular chamber in cross-section inside the annular box. The apparatus has provided uniform microwave radiation, effectively improved uniformity of heating and increased versatility. Furthermore, its structure is compact that occupies less space, and is convenient for material loading and thus facilitates a continuous production.
The above existing microwave drying devices have shortcomings that the material drying time is still very long, or the material is required to be injected into the drying devices in batches such that the material could not be fed continuously.